A semiconductor integrated circuit contains many layers of different materials usually classified according to whether the layer is a semiconductor, a dielectric (electrical insulator) or metal. However, some materials such as barrier materials, for example, TiN, are not so easily classified. The two principal current means of depositing metals and barrier materials are sputtering, also referred to as physical vapor deposition (PVD), and chemical vapor deposition (CVD). Of the two, sputtering has the inherent advantages of low cost source material and high deposition rates. However, sputtering has an inherent disadvantage when a material needs to filled into a deep narrow hole, that is, one having a high aspect ratio, or coated onto the sides of the hole, which is often required for barrier materials. Sputtering itself is primarily a nearly isotropic process producing ballistic sputter particles which do not easily reach the bottom of deep narrow holes. On the other hand, CVD tends to be a conformal process equally effective at the bottom of holes and on exposed top planar surfaces.
The conventional sputter reactor has a planar target in parallel opposition to the wafer being sputter deposited. A negative DC voltage is applied to the target sufficient to ionize the argon working gas into a plasma. The positive argon ions are attracted to the negatively charged target with sufficient energy to sputter atoms of the target material. Some of the sputtered atoms strike the wafer and form a sputter coating thereon. Most usually a magnetron is positioned in back of the target to create a magnetic field adjacent to the target. The magnetic field traps electrons, and, to maintain charge neutrality in the plasma, the ion density increases also. As a result, the plasma density and sputter rate are increased. The conventional magnetron generates a magnetic field principally lying parallel to the target.
Much effort has been expended to allow sputtering to effectively coat metals and barrier materials deep into narrow holes. High-density plasma (HDP) sputtering has been developed in which the argon working gas is excited into a high-density plasma, which is defined as a plasma having an ionization density of at least 10.sup.11 cm.sup.-3 across the entire space the plasma fills except the plasma sheath. Typically, an HDP sputter reactor uses an RF power source connected to an inductive coil adjacent to the plasma region to generate the high-density plasma. The high argon ion density causes a significant fraction of sputtered atoms to be ionized. If the pedestal electrode supporting the wafer being sputter coated is negatively electrically biased, the ionized sputter particles are accelerated toward the wafer to form a directional beam that reaches deeply into narrow holes.
HDP sputter reactors, however, have disadvantages. They involve a relatively new technology and are relatively expensive. Furthermore, the quality of the sputtered films they produce is often not the best, typically having an undulatory surface. Also, high-energy sputter ions tend to damage the material already deposited.
Another sputtering technology, referred to as self-ionized plasma (SIP) sputtering, has been developed to fill deep holes. See, for example, U.S. patent application Ser. No. 09/373,097 filed Aug. 12, 1999 by Fu and U.S. patent application filed Oct. 8, 1999 by Chiang et al. Both of these patent applications are incorporated by reference in their entireties. In its original implementations, SIP relies upon a somewhat standard capacitively coupled plasma sputter reactor having a planar target in parallel opposition to the wafer being sputter coated and a magnetron positioned in back of the target to increase the plasma density and hence the sputtering rate. The SIP technology, however, is characterized by a high target power density, a small magnetron, and a magnetron having an outer magnetic pole piece enclosing an inner magnetic pole piece with the outer pole piece having a significantly higher total magnetic flux than the inner pole piece. In some implementations, the target is separated from the wafer by a large distance to effect long-throw sputtering, which enhances collimated sputtering. The asymmetric magnetic pole pieces causes the magnetic field to have a significant vertical component extending far towards the wafer, thus enhancing extending the high-density plasma volume and promoting transport of ionized sputter particles.
The SIP technology was originally developed for sustained self-sputtering (SSS) in which a sufficiently high number of sputter particles are ionized that they may be used to further sputter the target and no argon working gas is required. Of the metals commonly used in semiconductor fabrication, only copper is susceptible to SSS resulting from its high self-sputtering yield.
The extremely low pressures and relatively high ionization fractions associated with SSS are advantageous for filling deep holes with copper. However, it was quickly realized that the SIP technology could be advantageously applied to the sputtering of aluminum and other metals and even to copper sputtering at moderate pressures. SIP sputtering produces high quality films exhibiting high hole filling factors regardless of the material being sputtered.
Nonetheless, SIP has some disadvantages. The small area of the magnetron requires circumferential scanning of the magnetron in a rotary motion at the back of the target. Even with rotary scanning, radial uniformity is difficult to achieve. Furthermore, very high target powers have been required in the previously known versions of SIP. High-capacity power supplies are expensive and necessitate complicated target cooling. Lastly, known versions of SIP tend to produce a relatively low ionization fraction of sputter particles, for example, 20%. The non-ionized fraction has a relatively isotropic distribution rather than forming a forward directed beam as the ionized particles are accelerated toward a biased wafer. Also, the target diameter is typically only slightly greater than the wafer diameter. As a result, those holes being coated located at the edge of the wafer have radially outer sidewalls which see a larger fraction of the wafer and are more heavily coated than the radially inner sidewalls. Therefore, the sidewalls of the edge holes are asymmetrically coated.
Other sputter geometries have been developed which increase the ionization density. One example is a multi-pole hollow cathode target, several variants of which are disclosed by Barnes et al. in U.S. Pat. No. 5,178,739. Its target has a hollow cylindrical shape, usually closed with a circular back wall, and is electrically biased. Typically, a series of magnets, positioned on the sides of the cylindrical cathode of alternating magnetic polarization, create a magnetic field extending generally parallel to the cylindrical sidewall. Helmer et al. in U.S. Pat. No. 5,482,611 disclose a hollow cathode target in which an axially polarized tubular magnet surrounds the sides of the hollow cathode and extend in back of the cathode back wall to create a generally axial magnetic field but which forms a cusp at the cathode back wall Another approach uses a pair of facing targets facing the lateral sides of the plasma space above the wafer. Such systems are described, for example, by Kitamoto et al. in "Compact sputtering apparatus for depositing Co--Cr alloy thin films in magnetic disks," Proceedings: The Fourth International Symposium on Sputtering & Plasma Processes, Kanazawa, Japan, Jun. 4-6, 1997, pp. 519-522, by Yamazato et al. in "Preparation of TiN thin films by facing targets magnetron sputtering, ibid., pp. 635-638, and by Musil et al. in "Unbalanced magnetrons and new sputtering systems with enhanced plasma ionization," Journal of Vacuum Science and Technology A, vol. 9, no. 3, May 1991, pp. 1171-1177. The facing pair geometry has the disadvantage that the magnets are stationary and create a horizontally extending field that is inherently non-uniform with respect to the wafer.
Musil et al., ibid., pp.1174, 1175 describe a coil-driven magnetic mirror magnetron having a central post of one magnetic polarization and surrounding rim of another polarization. An annular trough-shaped target is placed between the two post and rim. This structure has the disadvantage that the soft magnetic material forming the two poles, particularly the central spindle, are exposed to the plasma during sputtering and inevitably contaminate the sputtered layer. Furthermore, the coil drive provides a substantially cylindrical geometry, which may not be desired in some situations. Also, the disclosure illustrates a relatively shallow geometry for the target trough, which does not take advantage of some possible beneficial effects for a concavely shaped target.
It is thus desired to combine many of the good benefits of the different plasma sputter reactors described above while avoiding their separate disadvantages.